Problem: What is the greatest common factor of $40$ and $25$ ? Another way to say this is: $\operatorname{gcf}(40, 25) = {?}$
Explanation: The greatest common factor is the largest number that is a factor of both $40$ and $25$ The factors of $40$ are $1$ $2$ $4$ $5$ $8$ $10$ $20$ , and $40$ The factors of $25$ are $1$ $5$ , and $25$ Thus, the greatest common factor of $40$ and $25$ is $5$. $\operatorname{gcf}(40, 25) = 5$